This invention relates to wave transmission and, more particularly, to circuit arrangements for automatically balancing hybrid networks.
Hybrid networks are commonly used in bidirectional signal transmission systems to couple signals from a bidirectional transmission path to incoming and outgoing unidirectional transmission paths, a typical example being in coupling a 2-wire telephone transmission path to a 4-wire telephone transmission arrangement. As is well known, in such arrangements it is necessary to balance the hybrid by employing a network having an impedance which substantially matches the impedance of the bidirectional transmission path. Otherwise, low transhybrid loss results which, in turn, typically results in unwanted signal reflections.
Bidirectional transmission paths of various lengths are employed in communications systems and, therefore, present a wide range of impedances. Consequently, it has been necessary to provide precision hybrid balance networks including various manual adjustments in order to match the impedance of the particular bidirectional path being connected to the hybrid. Such manual adjustments rely heavily on knowledge of the characteristics of the particular bidirectional transmission path being connected to the hybrid. Needless to say, such knowledge is not always readily available. Moreover, the manual adjustment can result in mismatch through human error.
More recently, automatic balancing arrangements have been proposed in an attempt at overcoming the limitations of the prior manually adjusted balance networks. To this end, an arrangement has been proposed which employs an impedance network having an adjustable scalar multiplier, i.e., magnitude adjustment, and either an adjustable real zero or an adjustable real pole in conjunction with fixed real poles and/or fixed real zeros. The balance network is connected to a predetermined port of a 4-port hybrid as are a bidirectional transmission path and two unidirectional transmission paths. A single frequency test signal is applied at one of the unidirectional paths to the hybrid and a sense circuit generates two control signals. A first control signal represents the difference between the magnitudes of signals developed at the hybrid ports connected to the bidirectional path and the balance network, while a second control signal represents the difference in phase between those signals. The first control signal is employed to adjust the scalar multiplier while the second control signal is employed to adjust either the pole or the zero depending on the particular balance network being employed. The control signal values are stored and employed to maintain the adjustments of the balance network. One limitation of such an arrangement is that the impedance adjustments have been, at best, optimized for a single frequency. As is well known, most communication paths operate over some band of frequencies, for example, the voice frequency band. Another limitation of this prior arrangement is that it can adjust only one pole or one zero. Moreover, the pole or zero adjustment control signal is merely a function of the phase difference between the sensed voltages at a single frequency. It will be apparent that use of such a pole or zero control signal results in less than an optimum impedance match. Additionally, in certain applications it is desirable to have both an adjustable pole and an adjustable zero. Thus, although the prior automatic balance network may function satisfactorily in some applications, it is undesirable for others, especially in those applications in which balance is desired over the frequency band of the particular communications path.